Application of force in electrochemical cells

ABSTRACT

The present invention relates to the application of a force to enhance the performance of an electrochemical cell. The force may comprise, in some instances, an anisotropic force with a component normal to an active surface of the anode of the electrochemical cell. In the embodiments described herein, electrochemical cells (e.g., rechargeable batteries) may undergo a charge/discharge cycle involving deposition of metal (e.g., lithium metal) on a surface of the anode upon charging and reaction of the metal on the anode surface, wherein the metal diffuses from the anode surface, upon discharging. The uniformity with which the metal is deposited on the anode may affect cell performance. For example, when lithium metal is redeposited on an anode, it may, in some cases, deposit unevenly forming a rough surface. The roughened surface may increase the amount of lithium metal available for undesired chemical reactions which may result in decreased cycling lifetime and/or poor cell performance. The application of force to the electrochemical cell has been found, in accordance with the invention, to reduce such behavior and to improve the cycling lifetime and/or performance of the cell.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/685,860, filed Aug. 24, 2017, now U.S. Pat. No. 10,320,027, andentitled “Application of Force in Electrochemical Cells,” which is acontinuation of U.S. patent application Ser. No. 14/576,709, now U.S.Pat. No. 9,780,404, filed Dec. 19, 2014, and entitled “Application ofForce in Electrochemical Cells,” which is a continuation of U.S. patentapplication Ser. No. 12/535,328, filed Aug. 4, 2009, now U.S. Pat. No.9,105,938, and entitled “Application of Force in Electrochemical Cells,”which claims the benefit of U.S. Provisional Patent Application No.61/086,329, tiled Aug. 5, 2008, and entitled “Application of Force inElectrochemical Cells,” each of which is incorporated herein byreference in its entirety for all purposes.

FIELD OF INVENTION

The present invention relates to electrochemical cells, and morespecifically, to systems and methods for improving the performance ofelectrochemical cells via the application of a force.

BACKGROUND

A typical electrochemical cell has a cathode and an anode whichparticipate in an electrochemical reaction. Some electrochemical cells(e.g., rechargeable batteries) may undergo a charge/discharge cycleinvolving stripping and deposition of metal (e.g., lithium metal) on thesurface of the anode accompanied by parasitic reactions of the metal onthe anode surface with other cell components (e.g., electrolytecomponents), wherein the metal can diffuse from the anode surface duringdischarge. The efficiency and uniformity of such processes can affectefficient functioning of the electrochemical cell. In some cases, one ormore surfaces of one or more electrodes may become uneven as theelectrochemical cell undergoes repeated charge/discharge cycles, oftendue to uneven redeposition of an ion dissolved in the electrolyte. Theroughening of one or more surfaces of one or more electrodes can resultin increasingly poor cell performance.

Accordingly, improved compositions and methods are needed.

SUMMARY OF THE INVENTION

The present invention relates generally to electrochemical cells, and,more specifically, to systems and methods for improving the performanceof electrochemical cells via the application of force. The subjectmatter of the present invention involves, in some cases, interrelatedproducts, alternative solutions to a particular problem, and/or aplurality of different uses of one or more systems and/or articles.

In certain embodiments, the invention relates to an electrochemicalcell. In one set of embodiments, an electrochemical cell comprising acathode, an anode comprising lithium as an anode active material, theanode having an active surface, and an electrolyte in electrochemicalcommunication with the cathode and the anode are provided. The cell maybe constructed and arranged to apply, during at least one period of timeduring charge and/or discharge of the cell, an anisotropic force with acomponent normal to the active surface of the anode.

In some instances, an electrochemical cell comprising a cathode, ananode, the anode having an active surface, and a non-solid electrolytein electrochemical communication with the cathode and the anode may beprovided. The cell may be constructed and arranged to apply, during atleast one period of time during charge and/or discharge of the cell, ananisotropic force with a component normal to the active surface of theanode.

In some embodiments, an article is described. The article can comprisean electrochemical cell comprising an inner volume, a first electrodeproximate the inner volume, an electrolyte proximate the firstelectrode, and a second electrode proximate the electrolyte. The articlecan also comprise an expanding element positioned within the innervolume of the electrochemical cell, and a constricting elementsurrounding at least a portion of the outside of the electrochemicalcell. In some cases, the constricting element is constructed andarranged to apply a force to an outermost surface of the electrochemicalcell. In some embodiments, the expanding element is constructed andarranged to apply a force radiating outward from the inner volume of theelectrochemical cell. In some cases, the force within the boundaries ofthe electrochemical cell deviates by less than about 30% of the medianforce within the boundaries electrochemical cell.

In one set of embodiments, an article comprising a plurality ofelectrochemical cells is described. The article can comprise a firstelectrochemical cell, a second electrochemical cell, and a constrictingelement surrounding at least portions of the first cell and the secondcell. In some embodiments, the constricting element can be constructedand arranged to apply a force defining a pressure of at least about 4.9Newtons/cm² to the first and second cells.

In some cases, an electrochemical cell comprising a cathode with anactive surface, an anode with an active surface, and an electrolyte inelectrochemical communication with the cathode and the anode may beprovided. The cathode and anode may have yield stresses, wherein theeffective yield stress of one of the cathode and anode is greater thanthe yield stress of the other, such that an anisotropic force appliednormal to the surface of one of the active surface of the anode and theactive surface of the cathode causes the surface morphology of one ofthe cathode and the anode to be affected.

In certain embodiments, the invention relates to methods of electricalenergy storage and use. In one set of embodiments, the method comprisesproviding an electrochemical cell comprising a cathode; an anodecomprising a lithium anode active material, the anode having an activesurface; and an electrolyte in electrochemical communication with thecathode and the anode. The method may further comprise applying, duringat least one period of time during charge and/or discharge of the cell,an anisotropic force with a component normal to the active surface ofthe anode.

In one set of embodiments, the method comprises providing anelectrochemical cell comprising a cathode; an anode, the anode having anactive surface; and a non-solid electrolyte in electrochemicalcommunication with the cathode and the anode. The method may furthercomprise applying, during at least one period of time during chargeand/or discharge of the cell, an anisotropic force with a componentnormal to the active surface of the anode.

Other aspects, embodiments and features of the invention will becomeapparent from the following detailed description when considered inconjunction with the accompanying drawings. The accompanying figures areschematic and are not intended to be drawn to scale. For purposes ofclarity, not every component is labeled in every figure, nor is everycomponent of each embodiment of the invention shown where illustrationis not necessary to allow those of ordinary skill in the art tounderstand the invention. All patent applications and patentsincorporated herein by reference are incorporated by reference in theirentirety. In case of conflict, the present specification, includingdefinitions, will control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention. Inthe figures:

FIG. 1 is a schematic illustration of an electrochemical cell, accordingto one set of embodiments;

FIG. 2 is a schematic illustration of an electrochemical cell, accordingto another set of embodiments;

FIG. 3 is a schematic illustration of an electrochemical cell, accordingto yet another set of embodiments;

FIG. 4 is an SEM micrograph of an anode after the application of ananisotropic force during charge and discharge;

FIG. 5 is an SEM micrograph of an anode after charge and discharge inthe absence of an isotropic force;

FIGS. 6A-6D include SEM micrographs of anodes after the application of(a) 0, (b) 49, (c) 73.5, and (d) 98 Newtons/cm² during charge anddischarge;

FIGS. 7A-7D include SEM micrographs of anodes after the application of(a) 0, (b) 49, (c) 73.5, and (d) 98 Newtons/cm² during charge anddischarge; and

FIG. 8 is a schematic illustration of an electrochemical cell stack,according to another embodiment.

DETAILED DESCRIPTION

The present invention relates to the application of a force to enhancethe performance of an electrochemical cell. A force, or forces, appliedto portions of an electrochemical cell as described in this applicationcan reduce irregularity or roughening of an electrode surface of thecell, improving performance.

The force may comprise, in some instances, an anisotropic force with acomponent normal to an active surface of the anode of theelectrochemical cell. In the embodiments described herein,electrochemical cells (e.g., rechargeable batteries) may undergo acharge/discharge cycle involving deposition of metal (e.g., lithiummetal or other active material as described below) on a surface of theanode upon charging and reaction of the metal on the anode surface,wherein the metal diffuses from the anode surface, upon discharging. Theuniformity with which the metal is deposited on the anode may affectcell performance. For example, when lithium metal is removed from and/orredeposited on an anode, it may, in some cases, result in an unevensurface, for example, upon redeposition it may deposit unevenly forminga rough surface. The roughened surface may increase the amount oflithium metal available for undesired chemical reactions which mayresult in decreased cycling lifetime and/or poor cell performance. Theapplication of force to the electrochemical cell has been found, inaccordance with the invention, to reduce such behavior and to improvethe cycling lifetime and/or performance of the cell.

Although the present invention can find use in a wide variety ofelectrochemical devices, an example of one such device is provided inFIG. 1 for illustrative purposes only. In FIG. 1, a general embodimentof an electrochemical cell can include a cathode, an anode, and anelectrolyte layer in electrochemical communication with the cathode andthe anode. In some cases, the cell also may comprise a containmentstructure. The components may be assembled, in some cases, such that theelectrolyte is placed between the cathode and anode in a stackedconfiguration. FIG. 1 illustrates an electrochemical cell of theinvention. In the embodiment shown, cell 10 includes a cathode 30 thatcan be formed on a substantially planar surface of substrate 20. Whilethe cathode and substrate in FIG. 1 are shown as having a planarconfiguration, other embodiments may include non-planar configurations,as will be discussed in more detail later. The cathode may comprise avariety of cathode active materials. As used herein, the term “cathodeactive material” refers to any electrochemically active speciesassociated with the cathode. For example, the cathode may comprise asulfur-containing material, wherein sulfur is the cathode activematerial. Other examples of cathode active materials are described morefully below. In some embodiments, cathode 30 comprises at least oneactive surface (e.g., surface 32). As used herein, the term “activesurface” is used to describe a surface of an electrode that is inphysical contact with the electrolyte and at which electrochemicalreactions may take place. An electrolyte 40 (e.g., comprising a porousseparator material) can be formed adjacent the cathode 30.

In some embodiments, electrolyte 40 may comprise a non-solidelectrolyte, which may or may not be incorporated with a porousseparator. As used herein, the term “non-solid” is used to refer tomaterials that are unable to withstand a static shear stress, and when ashear stress is applied, the non-solid experiences a continuing andpermanent distortion. Examples of non-solids include, for example,liquids, deformable gels, and the like. An anode layer 50 can be formedadjacent electrolyte 40 and may be in electrical communication with thecathode 30. Optionally, the cell may also include, in some embodiments,containment structure 56.

The anode may comprise a variety of anode active materials. As usedherein, the term “anode active material” refers to any electrochemicallyactive species associated with the anode. For example, the anode maycomprise a lithium-containing material, wherein lithium is the anodeactive material. Other examples of anode active materials are describedmore fully below. In some embodiments, anode 50 comprises at least oneactive surface (e.g., surface 52). The anode 50 may also be formed on anelectrolyte layer positioned on cathode 30 via electrolyte 40. Ofcourse, the orientation of the components can be varied, and it shouldbe understood that there are other embodiments in which the orientationof the layers is varied such that, for example, the anode layer or theelectrolyte layer is first formed on the substrate. Optionally,additional layers (not shown), such as a multi-layer structure thatprotects an electroactive material (e.g., an electrode) from theelectrolyte, may be present, as described in more detail in U.S. patentapplication Ser. No. 11/400,781, filed Apr. 6, 2006, entitled,“Rechargeable Lithium/Water, Lithium/Air Batteries” to Affinito et al.,which is incorporated herein by reference in its entirety. Additionally,non-planar arrangements, arrangements with proportions of materialsdifferent than those shown, and other alternative arrangements areuseful in connection with the present invention. A typicalelectrochemical cell also would include, of course, current collectors,external circuitry, housing structure, and the like. Those of ordinaryskill in the art are well aware of the many arrangements that can beutilized with the general schematic arrangement as shown in the figuresand described herein.

While FIG. 1 illustrates an electrolytic cell arranged in a stackedconfiguration, it is to be understood that any electrochemical cellarrangement can be constructed, employing the principles of the presentinvention, in any configuration. For example, FIG. 2 illustrates across-sectional view of an electrochemical cell arranged as a cylinder.In the embodiment shown in FIG. 2, cell 100 includes an electrode 130,an electrolyte 140, and electrode 150. In some embodiments, electrode130 may comprise an anode while electrode 150 may comprise a cathode,while in other embodiments, their order may be reversed. Optionally, thecell may contain a core 170, which may be solid, hollow, or contain achannel or channels. Cell 100 also includes active surfaces 132 and 152.Optionally, the cell may also include, in some embodiments, containmentstructure 156. As shown in FIG. 2, electrode 130 is formed on core 170,electrolyte 140 is formed on electrode 130, and electrode 150 is formedon electrolyte 140. However, in some embodiments, electrode 130 may beproximate core 170, electrolyte 140 may be proximate electrode 130,and/or electrode 150 may be proximate electrolyte 140, optionallyincluding one or more intervening sections of material betweencomponents. In one set of embodiments, electrode 130 may at leastpartially surround core 170, electrolyte 140 may at least partiallysurround electrode 130, and/or electrode 150 may at least partiallysurround electrolyte 140. As used herein, a first entity is “at leastpartially surrounded” by a second entity if a closed loop can be drawnaround the first entity through only the second entity, and does notimply that the first entity is necessarily completely encapsulated bythe second entity.

In another set of embodiments, illustrated in FIG. 3, theelectrochemical cell is in the shape of a folded stack. The cell 200illustrated in FIG. 3 comprises electrolyte 240 separating anode 230 andcathode 250. The electrochemical cell in FIG. 3 comprises an electrolyteincluding three folded planes parallel to arrow 260. In otherembodiments, electrochemical cells may comprise electrolytes includingany number of folded planes parallel to arrow 260. Optionally, the cellmay also include, in some embodiments, containment structure 256. Inaddition to the shapes illustrated in FIGS. 1-3, the electrochemicalcells described herein may be of any other shape including, but notlimited to, prisms (e.g., triangular prisms, rectangular prisms, etc.),“Swiss-rolls,” non-planar stacks, etc. Additional configurations aredescribed in U.S. patent application Ser. No. 11/400,025, filed Apr. 6,2006, entitled, “Electrode Protection in both Aqueous and Non-AqueousElectrochemical Cells, including Rechargeable Lithium Batteries,” toAffinito et al., which is incorporated herein by reference in itsentirety.

As mentioned above, in some embodiments, the present invention relatesto electrochemical devices in which the application of force is used toenhance the performance of the device. In some embodiments, the forcecomprises an anisotropic force with a component normal to the activesurface of the anode. In the case of a planar surface, the force maycomprise an anisotropic force with a component normal to the surface atthe point at which the force is applied. For example, referring to FIG.1, a force may be applied in the direction of arrow 60. Arrow 62illustrates the component of the force that is normal to active surface52 of anode 50. In the case of a curved surface, for example, a concavesurface or a convex surface, the force may comprise an anisotropic forcewith a component normal to a plane that is tangent to the curved surfaceat the point at which the force is applied. Referring to the cylindricalcell illustrated in FIG. 2, a force may be applied to an externalsurface of the cell in the direction of, for example, arrow 180. In someembodiments, the force may be applied from the interior of thecylindrical cell, for example, in the direction of arrow 182. In someembodiments, an anisotropic force with a component normal to the activesurface of the anode is applied during at least one period of timeduring charge and/or discharge of the electrochemical cell. In someembodiments, the force may be applied continuously, over one period oftime, or over multiple periods of time that may vary in duration and/orfrequency. The anisotropic force may be applied, in some cases, at oneor more pre-determined locations, optionally distributed over the activesurface of the anode. In some embodiments, the anisotropic force isapplied uniformly over the active surface of the anode.

An “anisotropic force” is given its ordinary meaning in the art andmeans a force that is not equal in all directions. A force equal in alldirections is, for example, internal pressure of a fluid or materialwithin the fluid or material, such as internal gas pressure of anobject. Examples of forces not equal in all directions include forcesdirected in a particular direction, such as the force on a table appliedby an object on the table via gravity. Another example of an anisotropicforce includes a force applied by a band arranged around a perimeter ofan object. For example, a rubber band or turnbuckle can apply forcesaround a perimeter of an object around which it is wrapped. However, theband may not apply any direct force on any part of the exterior surfaceof the object not in contact with the band. In addition, when the bandis expanded along a first axis to a greater extent than a second axis,the band can apply a larger force in the direction parallel to the firstaxis than the force applied parallel to the second axis.

A force with a “component normal” to a surface, for example an activesurface of an anode, is given its ordinary meaning as would beunderstood by those of ordinary skill in the art and includes, forexample, a force which at least in part exerts itself in a directionsubstantially perpendicular to the surface. For example, in the case ofa horizontal table with an object resting on the table and affected onlyby gravity, the object exerts a force essentially completely normal tothe surface of the table. If the object is also urged laterally acrossthe horizontal table surface, then it exerts a force on the table which,while not completely perpendicular to the horizontal surface, includes acomponent normal to the table surface. Those of ordinary skill canunderstand other examples of these terms, especially as applied withinthe description of this document.

In some embodiments, the anisotropic force can be applied such that themagnitude of the force is substantially equal in all directions within aplane defining a cross-section of the electrochemical cell, but themagnitude of the forces in out-of-plane directions is substantiallyunequal to the magnitudes of the in-plane forces. For example, referringto FIG. 2, a cylindrical band may be positioned around the exterior ofcell 100 such that forces (e.g., force 180) are applied to the celloriented toward the cell's central axis (indicated by point 190 andextending into and out of the surface of the cross-sectional schematicdiagram). In some embodiments, the magnitudes of the forces orientedtoward the central axis of the cell are different (e.g., greater than)the magnitudes of the forces applied in out of plane directions (e.g.,parallel to central axis 190).

In one set of embodiments, cells of the invention are constructed andarranged to apply, during at least one period of time during chargeand/or discharge of the cell, an anisotropic force with a componentnormal to the active surface of the anode. Those of ordinary skill inthe art will understand the meaning of this. In such an arrangement, thecell may be formed as part of a container which applies such a force byvirtue of a “load” applied during or after assembly of the cell, orapplied during use of the cell as a result of expansion and/orcontraction of one or more portions of the cell itself.

The magnitude of the applied force is, in some embodiments, large enoughto enhance the performance of the electrolytic cell. The anode activesurface and the anisotropic force may be, in some instances, togetherselected such that the anisotropic force affects surface morphology ofthe anode active surface to inhibit increase in anode active surfacearea through charge and discharge and wherein, in the absence of theanisotropic force but under otherwise essentially identical conditions,the anode active surface area is increased to a greater extent throughcharge and discharge cycles. “Essentially identical conditions,” in thiscontext, means conditions that are similar or identical other than theapplication and/or magnitude of the force. For example, otherwiseidentical conditions may mean a cell that is identical, but where it isnot constructed (e.g., by brackets or other connections) to apply theanisotropic force on the subject cell.

The anode active surface and anisotropic force can be selected together,to achieve results described herein, easily by those of ordinary skillin the art. For example, where the anode active surface is relativelysofter, the component of the force normal to the anode active surfacemay be selected to be lower. Where the anode active surface is harder,the component of the force normal to the active surface may be greater.Those of ordinary skill in the art can easily select anode materials,alloys, mixtures, etc. with known or predictable properties, or readilytest the hardness or softness of such surfaces, and readily select cellconstruction techniques and arrangements to provide appropriate forcesto achieve what is described herein. Simple testing can be done, forexample by arranging a series of active materials, each with a series offorces applied normal (or with a component normal) to the activesurface, to determine the morphological effect of the force on thesurface without cell cycling (for prediction of the selected combinationduring cell cycling) or with cell cycling with observation of a resultrelevant to the selection.

In some embodiments, an anisotropic force with a component normal to theactive surface of the anode is applied, during at least one period oftime during charge and/or discharge of the cell, to an extent effectiveto inhibit an increase in surface area of the anode active surfacerelative to an increase in surface area absent the anisotropic force.The component of the anisotropic force normal to the anode activesurface may, for example, define a pressure of at least about 4.9, atleast about 9.8, at least about 24.5, at least about 49, at least about98, at least about 117.6, or at least about 147 Newtons per squarecentimeter. In some embodiments, the component of the anisotropic forcenormal to the anode active surface may, for example, define a pressureof less than about 196, less than about 147, less than about 117.6, lessthan about 98, less than about 49, less than about 24.5, or less thanabout 9.8 Newtons per square centimeter. In some cases, the component ofthe anisotropic force normal to the anode active surface is may define apressure of between about 4.9 and about 147 Newtons per squarecentimeter, between about 49 and about 117.6 Newtons per squarecentimeter, or between about 68.6 and about 98 Newtons per squarecentimeter. While forces and pressures are generally described herein inunits of Newtons and Newtons per unit area, respectively, forces andpressures can also be expressed in units of kilograms-force andkilograms-force per unit area, respectively. One or ordinary skill inthe art will be familiar with kilogram-force-based units, and willunderstand that 1 kilogram-force is equivalent to about 9.8 Newtons.

In some cases, one or more forces applied to the cell have a componentthat is not normal to an active surface of an anode. For example, inFIG. 1, force 60 is not normal to anode active surface 52, and force 60includes component 64, which is substantially parallel to anode activesurface 52. In addition, a force 66, which is substantially parallel toanode active surface 52, could be applied to the cell in some cases. Inone set of embodiments, the sum of the components of all appliedanisotropic forces in a direction normal to the anode active surface islarger than any sum of components in a direction that is non-normal tothe anode active surface. In some embodiments, the sum of the componentsof all applied anisotropic forces in a direction normal to the anodeactive surface is at least about 5%, at least about 10%, at least about20%, at least about 35%, at least about 50%, at least about 75%, atleast about 90%, at least about 95%, at least about 99%, or at leastabout 99.9% larger than any sum of components in a direction that isparallel to the anode active surface.

In some embodiments, the cathode and anode have yield stresses, whereinthe effective yield stress of one of the cathode and anode is greaterthan the yield stress of the other, such that an anisotropic forceapplied normal to the surface of one of the active surface of the anodeand the active surface of the cathode causes the surface morphology ofone of the cathode and the anode to be affected. In some embodiments,the component of the anisotropic force normal to the anode activesurface is between about 20% and about 200% of the yield stress of theanode material, between about 50% and about 120% of the yield stress ofthe anode material, or between about 80% and about 100% of the yieldstress of the anode material.

The anisotropic force described herein may be applied using any methodknown in the art. In some embodiments, the force may be applied usingcompression springs. For example, referring to FIG. 1, electrolytic cell10 may be situated in an optional enclosed containment structure 56 withone or more compression springs situated between surface 54 and theadjacent wall of the containment structure to produce a force with acomponent in the direction of arrow 62. In some embodiments, the forcemay be applied by situating one or more compression springs outside thecontainment structure such that the spring is located between an outsidesurface 58 of the containment structure and another surface (e.g., atabletop, the inside surface of another containment structure, anadjacent cell, etc.). Forces may be applied using other elements (eitherinside or outside a containment structure) including, but not limited toBelleville washers, machine screws, pneumatic devices, and/or weights,among others. For example, in one set of embodiments, one or more cells(e.g., a stack of cells) are arranged between two plates (e.g., metalplates). A device (e.g., a machine screw, a spring, etc.) may be used toapply pressure to the ends of the cell or stack via the plates. In thecase of a machine screw, for example, the cells may be compressedbetween the plates upon rotating the screw. As another example, in someembodiments, one or more wedges may be displaced between a surface ofthe cell (or the containment structure surrounding the cell) and a fixedsurface (e.g., a tabletop, the inside surface of another containmentstructure, an adjacent cell, etc.). The anisotropic force may be appliedby driving the wedge between the cell and the adjacent fixed surfacethrough the application of force on the wedge (e.g., by turning amachine screw).

In some cases, cells may be pre-compressed before they are inserted intocontainment structures, and, upon being inserted to the containmentstructure, they may expand to produce a net force on the cell. Forexample, the cylindrical cell of FIG. 2 could be pre-compressed andinserted within containment structure 156. The containment structurecould then provide a force to the outside surface of the cylindricalcell upon expansion of the cell. Such an arrangement may beadvantageous, for example, if the cylindrical cell is capable ofwithstanding relatively high variations in pressure. In suchembodiments, the containment structures may comprise a relatively highstrength (e.g., at least about 100 MPa, at least about 200 MPa, at leastabout 500 MPa, or at least about 1 GPa). In addition, the containmentstructure may comprise a relatively high elastic modulus (e.g., at leastabout 10 GPa, at least about 25 GPa, at least about 50 GPa, or at leastabout 100 GPa). The containment structure may comprise, for example,aluminum, titanium, or any other suitable material.

In some cases, any of the forces described herein may be applied to aplurality of electrochemical cells in a stack. As used herein, a “stack”of electrochemical cells is used to refer to a configuration in whichmultiple cells are arranged in an essentially cell-repetitive pattern,e.g., positioned on top of one another. In some cases, the cells may bepositioned such that at least one surface of each cell in the stack issubstantially parallel to at least one surface of every other cell inthe stack, e.g., where a surface of one particular component (e.g., theanode) of one cell is substantially parallel to the same surface of thesame component of every other cell. For example, FIG. 8 includes aschematic illustration of a stack of electrochemical cells 10. In someembodiments, the cells may be in direct contact with one another, whilein some instances one or more spacers may be positioned between thecells in a stack. The stack of electrochemical cells may comprise anynumber of cells (e.g., at least 2, at least 3, at least 5, at least 10,at least 25, at least 100 cells, or more).

In some embodiments, a constricting element may surround at least aportion of a cell or a stack of cells. The constricting element may beconstructed and arranged, in some cases, to apply an anisotropic forcewith a component normal to at least one anode active surface within thecell or stack of cells defining a pressure of at least about 4.9, atleast about 9.8, at least about 24.5, at least about 49, at least about98, at least about 117.6, at least about 147, less than about 196, lessthan about 147, less than about 117.6, less than about 98, less thanabout 49, less than about 24.5, less than about 9.8, between about 4.9and about 147, between about 49 and about 117.6, or between about 68.6and about 98 Newtons per square centimeter.

In some embodiments, the constricting element may comprise a band (e.g.,a rubber band, a turnbuckle band, etc.). An exemplary embodimentemploying a constricting element is illustrated in FIG. 8. In this setof embodiments, a constricting element 320 surrounds a stack of cells10. In some embodiments, a band can be affixed to the cell or stack ofcells by, for example adhesive, staples, clamps, a turn-buckle, or anyother suitable method, In some cases, the band comprises a turnbuckleband (e.g., a Kevlar turnbuckle band), and force is applied bytightening the band and securing the turnbuckle. In some instances, theband is a continuous elastic band. In some cases, after the elastic bandis stretched and positioned around the cell(s), a force may be appliedvia the elastic constriction of the band. As a specific example, a bandcan be installed by cooling the band material below its martensitictransformation temperature and plastically deforming (e.g., viastretching) the band to fit over the cell or stack of cells, Uponreturning to operating temperature, the band could then shrink to itspre-formed shape, by which the band could apply a force.

The constricting element may comprise any material with an amount ofelasticity necessary to produce the desired force. A solid band ofelastic material can be sized such that it provides required externalpressure upon being applied to the outer surface of the cell or cellsand relaxing. In some cases, the constricting element may comprise apolymeric material. The constricting element may comprise, for example,Desmopan® 392 (a polyester urethane, made by Bayer MaterialScience,Leverkusen, Germany), Estane® (an engineered polymer made by TheLubrizol Corporation, Wickliffe, Ohio), Kevlar® (a synthetic fiber madeby DuPont, Wilmington, Del.), among others. In some embodiments, theconstricting element may comprise a shape memory alloy (e.g., nitinol(NiTi)), which may expand and contract upon varying the temperature towhich the material is exposed. In some cases, the constricting elementcan comprise shrink wrap tubing such as, for example, polyester filmand/or fabric.

In some embodiments, the mass density of the elements used to apply aforce to a cell or a stack of cells (e.g., a constricting element, anexpanding element, etc.) is relatively low. By using elements withrelatively low mass densities, the energy density and specific energy ofthe cell or stack of cells may remain relatively high, In someembodiments the mass density of the article(s) used to apply a force toa cell or a stack of cells is less than about 10 g/cm³, less than about5 g/cm³, less than about 3 g/cm³, less than about 1 g/cm³, less thanabout 0.5 g/cm³, less than about 0.1 g/cm³, between about 0.1 g/cm³ andabout 10 g/cm³, between about 0.1 g/cm³ and about 5 g/cm³, or betweenabout 0.1 g/cm³ and about 3 g/cm³.

In some embodiments, pressure distribution components may be includedbetween a cell and another cell or between a cell and a constrictingelement. Such pressure distribution components can allow for a uniformforce to be applied throughout the cell or stack of cells. In somecases, the pressure distribution components comprise an end cap. The endcaps' shape can be selected so as to convert the linear forces appliedby the band to a uniform force across, for example, the active area ofan anode. For example, in FIG. 8, optional caps 310 may be placedbetween the ends of the stack and the band. The caps shown in FIG. 8include rounded ends, which may, for example, be used to reduceseparation of the band from the stack at corners and edges and enhancethe uniformity of the distribution of force. The caps can comprise anysuitable material including, for example, metal (e.g., aluminum), carbonfiber, plastics, etc. In some embodiments, the end caps are relativelyeasy to form or machine into complex shapes.

In some embodiments, the mass density of the end caps may be relativelylow. For example, the end caps may have a mass density of less thanabout 5 g/cm³, less than about 3 g/cm³, less than about 1 g/cm³, lessthan about 0.5 g/cm³, less than about 0.1 g/cm³, between about 0.1 g/cm³and about 10 g/cm³, between about 0.1 g/cm³ and about 5 g/cm³, orbetween about 0.1 g/cm³ and about 3 g/cm³. In addition, the end caps maycomprise any suitable stiffness. For example, the stiffness of the endcaps may be higher than 50 GPa, in some embodiments.

Another example of a pressure distribution component comprises a spacerpositioned between two cells. Inter-cell spacers can serve to reducestress concentrations that may arise, for example, due to geometricalmanufacturing variations of individual cells. For example, the flatnessof the cells may vary from cell to cell. As another example, opposingsides of one or more cells may not be perfectly parallel in some cases.In the set of embodiments illustrated in FIG. 8, optional spacers 330have been inserted between cells 10. Spacers can comprise any suitablematerial including, for example, metal (e.g., aluminum), metal foams,carbon composites, carbon foams, plastics, etc. In some embodiments, thespacers are relatively easy to form or machine into complex shapes.

A spacer can also have any suitable thickness. In some cases, a spacermay have an average thickness of less than about 10 mm, less than about5 mm, less than about 1 mm, less than about 500 microns, or less thanabout 250 microns. In some embodiments, a spacer can be between about100 microns and about 10 mm, between about 100 microns and about 1 mm,between about 250 microns and about 10 mm, between about 250 microns andabout 1 mm, or between about 500 microns and about 2 mm.

Opposing faces of the spacer(s) may be highly parallel, in someembodiments. For example, in some embodiments, the variation of thedistance between a first surface of a spacer in contact with a firstcell and a second surface of the spacer in contact with a second cell,as measured substantially parallel to a vector drawn from the center ofmass of the first cell to the center of mass of the second cell (e.g.,as in line 340 in FIG. 8), across the width of the spacer is less thanabout 1 mm, less than about 500 microns, less than about 100 microns,less than about 50 microns, less than about 25 microns, less than about10 microns, or less than about 1 micron.

The mass density of the spacer(s) in a stack of cells can be relativelylow, in some instances. For example, the spacers may have a mass densityof less than about 5 g/cm³, less than about 2 g/cm³, less than about 1g/cm³, less than about 0.5 g/cm³, less than about 0.1 g/cm³, betweenabout 0.1 g/cm³ and about 10 g/cm³, between about 0.1 g/cm³ and about 5g/cm³, or between about 0.1 g/cm³ and about 2 g/cm³. In addition, theend caps may comprise a relatively high stiffness. For example, thestiffness of the spacer(s) may be higher than 10 GPa, in someembodiments.

One of ordinary skill in the art will be able to perform experiments todetermine appropriate sizes, shapes, and materials of construction forthe end cap(s) or spacer(s) to be used with a cell or stack of cells.For example, if the end cap or spacer material is sufficiently stiff, asimple geometric optimization of the shape may be sufficient todetermine their properties. In other cases, more complex stress/straincalculations may be required to ensure the pressure distribution issubstantially uniform after the end caps and/or spacers haveequilibrated to their final deformed shape.

The use of constriction elements is not limited to flat cell geometries.In some instances, a constriction element may be used to apply a forceto a cylindrical electrochemical cell or a prismatic electrochemicalcell (e.g., a triangular prism, a rectangular prism, etc.). For example,in the set of embodiments in FIG. 2, optional constricting element 181can be positioned around the cell such that it surrounds at least aportion of the outside of the electrochemical cell. The constrictionelement may be used to apply a force to the outermost surface of theelectrochemical cell (e.g., surface 182 or surface 184 in FIG. 2).

Any of the constriction elements described above may be used asconstriction elements in cylindrical cells, prismatic cells, or othersuch cells. For example, in some embodiments, one or more wraps of thesame or different winding material may be positioned on the outsidesurface of the cell. In some embodiments, the winding material comprisesrelatively high strength. The winding material may also comprise arelatively high elastic modulus. In some cases, shrink wrap tubing suchas polyester film and fabric. In some cases, the constriction elementcomprises an elastic material properly sized to provide requiredexternal pressure after it relaxes on the outer surface of the cell.

In some embodiments, the cell may comprise an expanding element (e.g.,an expanding mandrel) within an inner volume of the cell such (e.g.,hollow core 170 in FIG. 2). The expanding element can be constructed andarranged to apply a force radiating outward from the inner volume of theelectrochemical cell, such as, for example, in the direction of arrow182 in FIG. 2, In some embodiments, the expanding element and theconstricting element can be constructed and arranged such that the forcewithin the boundaries of the electrochemical cell deviates by less thanabout 30%, less than about 20%, less than about 10%, or less than about5% of the median force within the boundaries electrochemical cell. Insome embodiments, such a distribution of forces can be achieved, forexample, by selecting constricting and expanding elements such thatsubstantially equal internal and external forces per unit area areapplied to the cell.

In some embodiments, rather than applying internal pressure, externalpressure application can be combined with complimentary windingmechanics to achieve a radial pressure distribution that is withinacceptable bounds. For example, proper surface nip winding (e.g., usinga nip roller) can produce a radial pressure distribution varying from107.9 Newtons/cm² at the inner diameter to 0 Newtons/cm² at the outerdiameter of the cell. The contracting element may be constructed andarranged to produce a force of 0 Newtons/cm² at the inner diameter and78.5 Newtons/cm² at the outer diameter. The superposition of these twodistributions can result in a mean pressure application of 98Newtons/cm² with a variation of ±19.6 Newtons/cm². In some embodiments,the total volumes of the pressure distribution elements(s) (e.g., endcaps, spacers, etc.) and the element(s) used to apply a force to thecell or stack of cells (e.g., bands, mandrels, etc.) may be relativelylow. By employing low volumes, the energy density of the assembly may bekept relatively high. In some cases, the sum of the volumes of thepressure distribution element(s) and the element(s) used to apply aforce to a cell or stack of cells comprises less than about 10%, lessthan about 5%, less than about 2%, less than about 1%, less than about0.5%, less than about 0.1%, between about 0.1% and about 10%, betweenabout 0.1% and about 5%, between about 0.1% and about 2%, or betweenabout 0.1% and about 1% of the volume of the cell or stack of cells.

In some cases, the cells described herein may change size (e.g., swell)during charge and discharge. When selecting the method of applying theanisotropic force, it may be desirable, in some embodiments, to selectmethods that produce a relatively constant force as the cell changesshape and/or size during charge and discharge. In some instances, thisselection may be analogous to selecting a system with a low effectivespring constant (e.g., a “soft” spring). For example, when using acompression spring to apply the anisotropic force, a spring with arelatively low spring constant may produce an anisotropic force that ismore constant during cell cycling than the force produced by a springwith a relatively high spring constant. In cases where elastic bands areused, a band with a relatively high elasticity may produce ananisotropic force that is more constant during cell cycling than theforce produced by a band with a relatively low elasticity. In someembodiments in which force is applied using a machine screw, the use ofsoft screws (e.g., brass, polymer, etc.) may be advantageous. In someapplications, for example, a machine screw may be selected to cover adesired range of compression, but the screw itself may be soft.

In some embodiments, the electrolytic cells of the present invention areplaced in containment structures, and at least a portion of ananisotropic force with a component normal to the active surface of theanode is produced due to the expansion of the electrolytic cell relativeto the containment structure. In some cases, the containment structureis sufficiently rigid such that it does not deform during the expansionof the electrolytic cell, resulting in a force applied on the cell. Theelectrolytic cell may swell as the result of a variety of phenomena. Forexample, in some cases, the electrolytic cell may undergo thermalexpansion. In some embodiments, the electrolytic cell may swell due tocharge and/or discharge of the cell. For example, in some cases, apartially discharged cell may be placed in a containment structure. Uponcharging the partially discharged cell, the cell may swell. Thisexpansion may be limited by the dimensions of the containment structure,resulting in the application of an anisotropic force.

In some cases, the cell may swell due to the adsorption of a liquid intoporous components of the electrolytic cell. For example, in someembodiments, a dry porous electrolytic cell may be placed within acontainment structure. The dry porous electrolytic cell may then besoaked (e.g., with a liquid electrolyte). In some cases, the propertiesof the electrolyte (e.g., surface tension) and the electrolytic cell(e.g., size of the porous cavities) may be selected such that, when theelectrolytic cell is wetted by the electrolyte, a desirable level ofcapillary pressure is generated. Once wetted, the electrode stack willswell, thus generating an anisotropic force. At equilibrium, theanisotropic force exerted by the containment structure on theelectrolytic cell will be equal to the force resulting from thecapillary pressure.

Containment structures described herein may comprise a variety of shapesincluding, but not limited to, cylinders, prisms (e.g., triangularprisms, rectangular prisms, etc.), cubes, or any other shape. In someembodiments, the shape of the containment structure is chosen such thatthe walls of the containment structure are parallel to the outersurfaces of the electrolytic cell. For example, in some cases, thecontainment structure may comprise a cylinder, which can be used, forexample, to surround and contain a cylindrical electrolytic cell. Inother instances, the containment structure may comprise a prismsurrounding a similarly shaped prismatic electrolytic cell.

In some embodiments, the invention relates to the discovery that theapplication of a force as described herein may allow for the use ofsmaller amounts of anode active material (e.g., lithium) and/orelectrolyte within an electrochemical cell, relative to the amounts usedin essentially identical cells in which the force is not applied. Incells lacking the applied force described herein, anode active material(e.g., lithium metal) may be, in some cases, redeposited unevenly on ananode during charge-discharge cycles of the cell, forming a roughsurface. In some cases, this may lead to an increase in the rates of oneor more undesired reactions involving the anode metal. These undesiredreactions may, after a number of charge-discharge cycles, stabilizeand/or begin to self-inhibit such that substantially no additional anodeactive material becomes depleted and the cell may function with theremaining active materials. For cells lacking the applied force asdescribed herein, this “stabilization” is often reached only after asubstantial amount of anode active material has been consumed and cellperformance has deteriorated. Therefore, in some cases where forces asdescribed herein have not been applied, a relatively large amount ofanode active material and/or electrolyte has often been incorporatedwithin cells to accommodate for loss of material during consumption ofactive materials, in order to preserve cell performance.

Accordingly, the application of force as described herein may reduceand/or prevent depletion of active materials such that the inclusion oflarge amounts of anode active material and/or electrolyte within theelectrochemical cell may not be necessary. For example, the force may beapplied to a cell prior to use of the cell, or in an early stage in thelifetime of the cell (e.g., less than five charge-discharge cycles),such that little or substantially no depletion of active material mayoccur upon charging or discharging of the cell. By reducing and/oreliminating the need to accommodate for active material loss duringcharge-discharge of the cell, relatively small amounts of anode activematerial may be used to fabricate cells and devices as described herein.In some embodiments, the invention relates to devices comprising anelectrochemical cell having been charged and discharged less than fivetimes in its lifetime, wherein the cell comprises an anode, a cathode,and an electrolyte, wherein the anode comprises no more than five timesthe amount of anode active material which can be ionized during one fulldischarge cycle of the cell. In some cases, the anode comprises no morethan four, three, two, or 1.5 times the amount of lithium which can beionized during one full discharge cycle of the cell.

In some cases, the present invention relates to devices comprising anelectrochemical cell, wherein the cell comprises an anode activematerial, a cathode active material, and an electrolyte, wherein theratio of the amount of anode active material in the anode to the amountof cathode active material in the cathode is less than about 5:1, lessthan about 3:1, less than about 2:1, or less than about 1.5:1 on a molarbasis. For example, a cell may comprise lithium as an anode activematerial and sulfur as an cathode active material, wherein the molarratio Li:S is less than about 5:1. In some cases, the molar ratio oflithium to sulfur, Li:S, is less than about 3:1, less than about 2:1, orless than about 1.5:1. In some embodiments, the ratio of anode activematerial (e.g., lithium) to cathode active material by weight may beless than 2:1, less than about 1.5:1, less than about 1.25:1, or lessthan about 1.1:1. For example, a cell may comprise lithium as the anodeactive material and sulfur as the cathode active material, wherein theratio Li:S by weight is less than about 2:1, less than about 1.5:1, lessthan about 1.25:1, or less than about 1.1:1.

The use of smaller amounts of anode active material and/or electrolytematerial may advantageously allow for electrochemical cells, or portionsthereof, having decreased thickness. In some embodiments, the anodelayer and the electrolyte layer together have a maximum thickness of 500microns. In some cases, the anode layer and the electrolyte layertogether have a maximum thickness of 400 microns, 300 microns, 200microns, or, in some cases, 100 microns.

In some embodiments, the application of force, as described herein, mayresult in improved capacity after repeated cycling of theelectrochemical cell. For example, in some embodiments, afteralternatively discharging and charging the cell three times, the cellexhibits at least about 50%, at least about 80%, at least about 90%, orat least about 95% of the cell's initial capacity at the end of thethird cycle. In some cases, after alternatively discharging and chargingthe cell ten times, the cell exhibits at least about 50%, at least about80%, at least about 90%, or at least about 95% of the cell's initialcapacity at the end of the tenth cycle. In still further cases, afteralternatively discharging and charging the cell twenty-five times, thecell exhibits at least about 50%, at least about 80%, at least about90%, or at least about 95% of the cell's initial capacity at the end ofthe twenty-fifth cycle.

Suitable electroactive materials for use as cathode active materials inthe cathode of the electrochemical cells of the invention include, butare not limited to, electroactive transition metal chalcogenides,electroactive conductive polymers, sulfur, carbon and/or combinationsthereof. As used herein, the term “chalcogenides” pertains to compoundsthat contain one or more of the elements of oxygen, sulfur, andselenium. Examples of suitable transition metal chalcogenides include,but are not limited to, the electroactive oxides, sulfides, andselenides of transition metals selected from the group consisting of Mn,V, Cr, Ti, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re,Os, and Ir. In one embodiment, the transition metal chalcogenide isselected from the group consisting of the electroactive oxides ofnickel, manganese, cobalt, and vanadium, and the electroactive sulfidesof iron. In one embodiment, a cathode includes one or more of thefollowing materials: manganese dioxide, iodine, silver chromate, silveroxide and vanadium pentoxide, copper oxide, copper oxyphosphate, leadsulfide, copper sulfide, iron sulfide, lead bismuthate, bismuthtrioxide, cobalt dioxide, copper chloride, manganese dioxide, andcarbon. In another embodiment, the cathode active layer comprises anelectroactive conductive polymer. Examples of suitable electroactiveconductive polymers include, but are not limited to, electroactive andelectronically conductive polymers selected from the group consisting ofpolypyrroles, polyanilines, polyphenylenes, polythiophenes, andpolyacetylenes. Examples of conductive polymers include polypyrroles,polyanilines, and polyacetylenes.

In some embodiments, electroactive materials for use as cathode activematerials in electrochemical cells described herein includeelectroactive sulfur-containing materials. “Electroactivesulfur-containing materials,” as used herein, relates to cathode activematerials which comprise the element sulfur in any form, wherein theelectrochemical activity involves the oxidation or reduction of sulfuratoms or moieties. The nature of the electroactive sulfur-containingmaterials useful in the practice of this invention may vary widely, asknown in the art. For example, in one embodiment, the electroactivesulfur-containing material comprises elemental sulfur. In anotherembodiment, the electroactive sulfur-containing material comprises amixture of elemental sulfur and a sulfur-containing polymer. Thus,suitable electroactive sulfur-containing materials may include, but arenot limited to, elemental sulfur and organic materials comprising sulfuratoms and carbon atoms, which may or may not be polymeric. Suitableorganic materials include those further comprising heteroatoms,conductive polymer segments, composites, and conductive polymers.

In some embodiments, the cathode may comprise one or more bindermaterials (e.g., polymers, porous silica sol-gel, etc.).

Examples of sulfur-containing polymers include those described in: U.S.Pat. Nos. 5,601,947 and 5,690,702 to Skotheim et al.; U.S. Pat. Nos.5,529,860 and 6,117,590 to Skotheim et al.; U.S. Pat. No. 6,201,100issued Mar. 13, 2001, to Gorkovenko et al. of the common assignee, andPCT Publication No. WO 99/33130. Other suitable electroactivesulfur-containing materials comprising polysulfide linkages aredescribed in U.S. Pat. No. 5,441,831 to Skotheim et al.; U.S. Pat. No.4,664,991 to Perichaud et al., and in U.S. Pat. Nos. 5,723,230,5,783,330, 5,792,575 and 5,882,819 to Naoi et al. Still further examplesof electroactive sulfur-containing materials include those comprisingdisulfide groups as described, for example in, U.S. Pat. No. 4,739,018to Armand et al.; U.S. Pat. Nos. 4,833,048 and 4,917,974, both to DeJonghe et al.; U.S. Pat. Nos. 5,162,175 and 5,516,598, both to Visco etal.; and U.S. Pat. No. 5,324,599 to Oyama et al.

In one embodiment, an electroactive sulfur-containing material of acathode active layer comprises greater than 50% by weight of sulfur. Inanother embodiment, the electroactive sulfur-containing materialcomprises greater than 75% by weight of sulfur. In yet anotherembodiment, the electroactive sulfur-containing material comprisesgreater than 90% by weight of sulfur.

The cathode active layers of the present invention may comprise fromabout 20 to 100% by weight of electroactive cathode materials (e.g., asmeasured after an appropriate amount of solvent has been removed fromthe cathode active layer and/or after the layer has been appropriatelycured). In one embodiment, the amount of electroactive sulfur-containingmaterial in the cathode active layer is in the range of 5-30% by weightof the cathode active layer. In another embodiment, the amount ofelectroactive sulfur-containing material in the cathode active layer isin the range of 20% to 90% by weight of the cathode active layer.

Non-limiting examples of suitable liquid media (e.g., solvents) for thepreparation of cathodes (as well as other components of cells describedherein) include aqueous liquids, non-aqueous liquids, and mixturesthereof. In some embodiments, liquids such as, for example, water,methanol, ethanol, isopropanol, propanol, butanol, tetrahydrofuran,dimethoxyethane, acetone, toluene, xylene, acetonitrile, cyclohexane,and mixtures thereof can be used. Of course, other suitable solvents canalso be used as needed.

Positive electrode layers may be prepared by methods known in the art.For example, one suitable method comprises the steps of: (a) dispersingor suspending in a liquid medium the electroactive sulfur-containingmaterial, as described herein; (b) optionally adding to the mixture ofstep (a) a conductive filler and/or binder; (c) mixing the compositionresulting from step (b) to disperse the electroactive sulfur-containingmaterial; (d) casting the composition resulting from step (c) onto asuitable substrate; and (e) removing some or all of the liquid from thecomposition resulting from step (d) to provide the cathode active layer.

In some embodiments, the use of a cathode that is resistant tocompression can enhance the performance of the cell relative to cells inwhich the cathode is significantly compressible. Not wishing to be boundby any theory, the use of elastic, relatively highly compressiblecathodes may result in the evacuation of liquid electrolyte during theapplication of the anisotropic force. The evacuation of liquidelectrolyte from the cathode may result in decreased power output duringthe operation of the electrolytic cell. For example, in some cases adecrease in power output from the electrolytic cell may be observed evenwhen the anisotropic force is relatively small (e.g., an anisotropicforce with a component normal to an active surface of the anode defininga pressure of about 68.6 Newtons/cm²) or when the anisotropic force isof another magnitude, for example, as noted above with reference tolimits and ranges of the component of the anisotropic force normal tothe anode active surface. The degree of compressibility can becorrelated to a change in porosity, i.e., change in void volume of thecathode, during application of a compressive force. In some embodiments,it may be desirable to limit the change in porosity of the cathodeduring the operation of the cell. For example, in some embodiments ofthe invention, the porosity of the cathode may be decreased duringoperation of the cell by less than 10%, less than 6%, less than 4%, lessthan 2%, less than 1%, less than 0.5%, less than 0.1%, or lower. Thatis, during use of the cell, a compressive force experienced by thecathode may reduce the total void volume, or total volume otherwiseaccessible by the electrolyte, by percentages noted above, where thecathode is fabricated to provide suitable resistance to compression.

The stiffness of the cathode (resistance to compressibility) may beenhanced using a variety of methods. In some embodiments, the type ofelectrolyte and the size of the pores in the cathode may be togetherselected such that the resulting capillary forces produced by theinteraction of the electrolyte and the cathode pores resist thedeformation of the cathode. This effect may be particularly useful, forexample, in small electrolytic cells. As another example, the stiffnessof the cathode may be enhanced by incorporating reinforcement fibers(e.g., to connect carbon particles) into the cathode. In some cases,binder may be incorporated into the cathode to provide rigidity. Inother embodiments, an inherently rigid cathode may be produced byinfusing active material (e.g., reticulated Ni foam) into a thin andlight superstructure.

Suitable electroactive materials for use as anode active materials inthe anode of the electrochemical cells described herein include, but arenot limited to, lithium metal such as lithium foil and lithium depositedonto a conductive substrate, and lithium alloys (e.g., lithium-aluminumalloys and lithium-tin alloys). While these are preferred negativeelectrode materials, the current collectors may also be used with othercell chemistries. In some embodiments, the anode may comprise one ormore binder materials (e.g., polymers, etc.).

Methods for depositing a negative electrode material (e.g., an alkalimetal anode such as lithium) onto a substrate may include methods suchas thermal evaporation, sputtering, jet vapor deposition, and laserablation. Alternatively, where the anode comprises a lithium foil, or alithium foil and a substrate, these can be laminated together by alamination process as known in the art to form an anode.

In one embodiment, an electroactive lithium-containing material of ananode active layer comprises greater than 50% by weight of lithium. Inanother embodiment, the electroactive lithium-containing material of ananode active layer comprises greater than 75% by weight of lithium. Inyet another embodiment, the electroactive lithium-containing material ofan anode active layer comprises greater than 90% by weight of lithium.

Positive and/or negative electrodes may optionally include one or morelayers that interact favorably with a suitable electrolyte, such asthose described in U.S. Provisional Application Ser. No. 60/872,939,filed Dec. 4, 2006 and entitled “Separation of Electrolytes,” byMikhaylik et al., which is incorporated herein by reference in itsentirety.

The electrolytes used in electrochemical or battery cells can functionas a medium for the storage and transport of ions, and in the specialcase of solid electrolytes and gel electrolytes, these materials mayadditionally function as a separator between the anode and the cathode.Any liquid, solid, or gel material capable of storing and transportingions may be used, so long as the material facilitates the transport ofions (e.g., lithium ions) between the anode and the cathode. Theelectrolyte is electronically non-conductive to prevent short circuitingbetween the anode and the cathode. In some embodiments, the electrolytemay comprise a non-solid electrolyte.

The electrolyte can comprise one or more ionic electrolyte salts toprovide ionic conductivity and one or more liquid electrolyte solvents,gel polymer materials, or polymer materials. Suitable non-aqueouselectrolytes may include organic electrolytes comprising one or morematerials selected from the group consisting of liquid electrolytes, gelpolymer electrolytes, and solid polymer electrolytes. Examples ofnon-aqueous electrolytes for lithium batteries are described by Dornineyin Lithium Batteries, New Materials, Developments and Perspectives,Chapter 4, pp. 137-165, Elsevier, Amsterdam (1994). Examples of gelpolymer electrolytes and solid polymer electrolytes are described byAlamgir et al. in Lithium Batteries, New Materials, Developments andPerspectives, Chapter 3, pp. 93-136, Elsevier, Amsterdam (1994).Heterogeneous electrolyte compositions that can be used in batteriesdescribed herein are described in U.S. Provisional Application Ser. No.60/872,939, filed Dec. 4, 2006.

Examples of useful non-aqueous liquid electrolyte solvents include, butare not limited to, non-aqueous organic solvents, such as, for example,N-methyl acetamide, acetonitrile, acetals, ketals, esters, carbonates,sulfones, sulfites, sulfolanes, aliphatic ethers, cyclic ethers, glymes,polyethers, phosphate esters, siloxanes, dioxolanes,N-alkylpyrrolidones, substituted forms of the foregoing, and blendsthereof. Fluorinated derivatives of the foregoing are also useful asliquid electrolyte solvents.

In some cases, aqueous solvents can be used as electrolytes for lithiumcells. Aqueous solvents can include water, which can contain othercomponents such as ionic salts. As noted above, in some embodiments, theelectrolyte can include species such as lithium hydroxide, or otherspecies rendering the electrolyte basic, so as to reduce theconcentration of hydrogen ions in the electrolyte.

Liquid electrolyte solvents can also be useful as plasticizers for gelpolymer electrolytes, i.e., electrolytes comprising one or more polymersforming a semi-solid network. Examples of useful gel polymerelectrolytes include, but are not limited to, those comprising one ormore polymers selected from the group consisting of polyethylene oxides,polypropylene oxides, polyacrylonitriles, polysiloxanes, polyimides,polyphosphazenes, polyethers, sulfonated polyimides, perfluorinatedmembranes (NAFION resins), polydivinyl polyethylene glycols,polyethylene glycol diacrylates, polyethylene glycol dimethacrylates,derivatives of the foregoing, copolymers of the foregoing, crosslinkedand network structures of the foregoing, and blends of the foregoing,and optionally, one or more plasticizers. In some embodiments, a gelpolymer electrolyte comprises between 10-20%, 20-40%, between 60-70%,between 70-80%, between 80-90%, or between 90-95% of a heterogeneouselectrolyte by volume.

In some embodiments, one or more solid polymers can be used to form anelectrolyte. Examples of useful solid polymer electrolytes include, butare not limited to, those comprising one or more polymers selected fromthe group consisting of polyethers, polyethylene oxides, polypropyleneoxides, polyimides, polyphosphazenes, polyacrylonitriles, polysiloxanes,derivatives of the foregoing, copolymers of the foregoing, crosslinkedand network structures of the foregoing, and blends of the foregoing.

In addition to electrolyte solvents, gelling agents, and polymers asknown in the art for forming electrolytes, the electrolyte may furthercomprise one or more ionic electrolyte salts, also as known in the art,to increase the ionic conductivity.

Examples of ionic electrolyte salts for use in the electrolytes of thepresent invention include, but are not limited to, LiSCN, LiBr, LiI,LiClO₄, LiAsF₆, LiSO₃CF₃, LiSO₃CH₃, LiBF₄, LiB(Ph)₄, LiPF₆,LiC(SO₂CF₃)₃, and LiN(SO₂CF₃)₂. Other electrolyte salts that may beuseful include lithium polysulfides (Li₂S_(x)), and lithium salts oforganic ionic polysulfides (LiS_(x)R)_(n), where x is an integer from 1to 20, n is an integer from 1 to 3, and R is an organic group, and thosedisclosed in U.S. Pat. No. 5,538,812 to Lee et al.

In some embodiments, electrochemical cells may further comprise aseparator interposed between the cathode and anode. The separator may bea solid non-conductive or insulative material which separates orinsulates the anode and the cathode from each other preventing shortcircuiting, and which permits the transport of ions between the anodeand the cathode. In some embodiments, the porous separator may beperrmeable to the electrolyte.

The pores of the separator may be partially or substantially filled withelectrolyte. Separators may be supplied as porous free standing filmswhich are interleaved with the anodes and the cathodes during thefabrication of cells. Alternatively, the porous separator layer may beapplied directly to the surface of one of the electrodes, for example,as described in PCT Publication No. WO 99/33125 to Carlson et al. and inU.S. Pat. No. 5,194,341 to Bagley et al.

A variety of separator materials are known in the art. Examples ofsuitable solid porous separator materials include, but are not limitedto, polyolefins, such as, for example, polyethylenes (e.g., SETELA™ madeby Tonen Chemical Corp) and polypropylenes, glass fiber filter papers,and ceramic materials. For example, in some embodiments, the separatorcomprises a microporous polyethylene film. Further examples ofseparators and separator materials suitable for use in this inventionare those comprising a microporous xerogel layer, for example, amicroporous pseudo-boehmite layer, which may be provided either as afree standing film or by a direct coating application on one of theelectrodes, as described in U.S. Pat. Nos. 6,153,337 and 6,306,545 byCarlson et al. of the common assignee. Solid electrolytes and gelelectrolytes may also function as a separator in addition to theirelectrolyte function.

The following applications are each incorporated herein by reference intheir entirety: U.S. Provisional Patent Application Ser. No. 61/086,329,filed Aug. 5, 2008, entitled “Application of Force in ElectrochemicalCells,” by Scordilis-Kelley, et al.

The following examples are intended to illustrate certain embodiments ofthe present invention, but do not exemplify the full scope of theinvention.

EXAMPLE 1

In this example, an anisotropic force with a component normal to theactive surface of an anode is applied to an electrochemical cell duringat least one period of time during charge and/or discharge of the cell.The anode active surface and the anisotropic force were togetherselected such that the anisotropic force affected the surface morphologyof the anode active surface to inhibit increase in anode active surfacearea through charge and discharge. As illustrated in Example 2, in theabsence of the anisotropic force but under otherwise essentiallyidentical conditions, the anode active surface area is increased to agreater extent through charge and discharge cycles.

In this example, lithium metal (>99.9% Li, 2 mil thick foil availablefrom Chemetall-Foote Corp., Kings Mountain, N.C.) was used as the anode.The electrolyte comprised 13.2 parts of lithium bis (trifluoromethanesulfonyl) imide, (lithium imide available from 3M Corporation, St. Paul,Minn.), 1.1 parts lithium nitrate (available from Aldrich ChemicalCompany, Milwaukee, Wis.) in 1,3-dioxolane, with water content of lessthan 50 ppm. The porous separator used was 16 μm SETELA (a trademark fora polyolefin separator available from Tonen Chemical Corporation, Tokyo,Japan, and also available from Exxon Mobil Chemical Company, FilmsDivision, Pittsford, N.Y.).

The above components were stacked into a layered structure ofanode/separator/anode, with the liquid electrolyte filling the voidareas of the separator to form prismatic cells with an electrode area ofabout 16 cm². After sealing in aluminized flexible packaging (fromSumitomo), the cells were stored for 24 hours and placed between steelplates with compression springs. The cell was constructed and arrangedto apply, during at least one period of time during charge and/ordischarge of the cell, an anisotropic force with a component normal tothe active surface of the anode. In this example, the anisotropic forcedefined a pressure of 98 Newtons/cm².

Discharge-charge cycling on these cells was performed 30 times at 18 mAfor 4 hours, both discharge and charge (constant capacity cyclingequivalent to 41.5% Li DOD). The cells were then disassembled and theelectrodes' morphology evaluated by a scanning electron microscope andresidual metallic lithium measured by differential scanning calorimetry.FIG. 4 includes an SEM micrograph of a lithium anode after theapplication of 98 Newtons/cm² over 30 cycles. The resulting anodes werecompact had retained their original thickness and were composedprimarily of lithium metal.

EXAMPLE 2

In this example, an electrochemical cell identical to the cell employedin Example 1 was charged and discharged. In this example, theelectrochemical cell was charged and discharged in the absence of theanisotropic force. In addition, the conditions under which the cell wasoperated were essentially identical to those outlined in Example 1.

Using the same analysis as in Example 1, it was found that, aftercycling in the absence of the anisotropic force but under otherwiseessentially identical conditions, the anode active surface area wasincreased to a greater extent through charge and discharge cycles. FIG.5 includes an SEM micrograph of a lithium anode after charge anddischarge in the absence of an isotropic force. The resulting anodeswere very porous, had more than doubled in thickness, and were composedprimarily of decomposition products rather than metallic lithium.

EXAMPLE 3

FIGS. 6A-D includes SEM micrographs of various deposited lithium metalanodes after 30 charge/discharge cycles. The anode in FIG. 6A was cycledin the absence of an anisotropic force, while the anodes in FIGS. 6B, C,and D were cycled with applied forces defining pressures of 49, 73.5,and 98 Newtons/cm², respectively. From the micrographs, it can be seenthat as the applied force was increased, the resulting lithium metalanode was thinner and less porous.

FIGS. 7A-D includes SEM micrographs of various stripped lithium metalanodes after 30 charge/discharge cycles. The anode in FIG. 7A was cycledin the absence of an anisotropic force, while the anodes in FIGS. 7B, C,and D were cycled with applied forces of 49, 73.5, and 98 Newtons/cm²,respectively. Again, as the applied force was increased, the resultinglithium metal anode was thinner and less porous.

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, kit, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,kits, and/or methods, if such features, systems, articles, materials,kits, and/or methods are not mutually inconsistent, is included withinthe scope of the present invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

What is claimed is:
 1. A method, comprising: at least partiallydischarging an electrochemical cell; and at least partially charging theelectrochemical cell; wherein: an anisotropic force comprising acomponent normal to an active surface comprising metallic lithium isapplied during at least one period of time during the discharging and/orthe charging of the electrochemical cell, and the component normal tothe active surface defines a pressure of at least 98 Newtons/cm².
 2. Themethod of claim 1, wherein the anisotropic force is applied uniformlyover the active surface.
 3. The method of claim 1, wherein the activesurface is part of a layer of lithium metal.
 4. The method of claim 1,wherein at least a portion of the anisotropic force is applied externalto a container in which the electrochemical cell is contained.
 5. Themethod of claim 1, wherein the anisotropic force is applied during atleast one period of time during the charging and at least one period oftime during the discharging.
 6. The method of claim 5, wherein theanisotropic force is applied between the charging and the discharging.7. The method of claim 1, wherein the anisotropic force is applied priorto a first charge of the electrochemical cell and prior to a firstdischarge of the electrochemical cell.
 8. The method of claim 1, whereinafter alternatively discharging and charging the electrochemical celltwenty-five times, the electrochemical cell exhibits at least about 50%of the initial capacity of the electrochemical cell at the end of thetwenty-fifth cycle.
 9. The method of claim 1, wherein the anisotropicforce is applied between the charging and the discharging.
 10. Themethod of claim 1, wherein the component normal to the active surfacedefines a pressure of at least 98 Newtons/cm² and less than 196Newtons/cm².
 11. The method of claim 1, wherein the electrochemical cellcomprises a solid electrolyte.
 12. The method of claim 1, wherein theelectrochemical cell comprises an electrode active material comprisingat least one transition metal chalcogenide, at least one conductivepolymer, sulfur, carbon, and/or combinations thereof.
 13. The method ofclaim 1, wherein the electrochemical cell comprises an electrode activematerial comprising an oxide, a sulfide, and/or a selenide of one ormore transition metals selected from the group consisting of Mn, V, Cr,Ti, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os,and Ir.
 14. The method of claim 1, wherein the electrochemical cellcomprises an electrode active material comprising an oxide of nickel, anoxide of manganese, an oxide of cobalt, an oxide of vanadium, and/or asulfide of iron.